Polymer blending is a versatile tool to combine the beneficial properties of two or more components in one single material. Here, we present the preparation, thermal- and mass transport properties of a series of blend membranes made from the commercially available PEBAX® MH 1657 and a poly(ethylene glycol) (PEG) based additive. The additive (PDMS–PEG) consists for 80 wt.% of PEG and the remaining 20 wt.% is poly(dimethyl siloxane) (PDMS), which is highly flexible and permeable. As such, we combine the high selectivity of PEG for CO2 with the high permeability of PDMS. We extensively study the gas transport properties, and in particular the CO2/light gas separation performance, using pure and mixed gases. The pure gas CO2 permeability in the PEBAX®1657/PDMS–PEG blend membranes increased by a factor 5 to approximately 530 Barrer at 50 wt.% PDMS–PEG loading. CO2 sorption measurements revealed only a 50% increase in CO2 solubility and the increase in permeability could be mainly attributed to an increase in diffusivity. Remarkably, the CO2/H2 selectivity is enhanced (∼10%) at 50 wt.% loading, while the CO2/N2 and CO2/CH4 selectivity only slightly decreases. Mixed gas CO2/H2 and CO2/CH4 permeation measurements up to CO2 partial pressures of 25 bar reveal that the CO2 permeability is slightly reduced when gas mixtures are used. The CO2/H2 selectivity was found to be independent of the PDMS–PEG loading and feed pressure at a value of approximately 9–10. CO2/CH4 mixed gas selectivity decreased slightly with PDMS–PEG loading and pressure, but always remained above 10.